One type of a conventional circuit for converting a phase signal to an amplitude signal is included, for instance, in a digital frequency modulator. The circuit for converting a phase signal to an amplitude signal has been described on pages 355 to 361 of "IEEE Transactions on Consumer Electronics, Vol. CE-32, No. 3, August 1986," and comprises a Sine look-up table ROM, in which amplitude data of one period sinusoidal wave at addresses of 0 to 2 .pi. are stored. In the digital frequency modulator, the look-up table ROM is specified to have an input bit width of 10 bits for an address bit width, and an output bit width of 8 bits for a data bit width, and a clock signal of approximately 20 MHz, by which an operating speed is determined in the processing of video signals, is used. The details of the digital frequency modulator will be explained later.
However, the circuit for converting a phase signal to an amplitude signal has a disadvantage in that the Sine look-up table ROM is of a large memory capacity as much as 8 Kbits. Furthermore, it has another disadvantage in that an access to the look up table ROM having the input bit width of 10 bits becomes difficult in the use of the 20 MHz clock signal.